This invention relates to a method for the testing of laundry detergent performance, more particularly to a method for simultaneously determining both the removal and subsequent redeposition of soils on fabrics during the laundry wash cycle.
At present there is great interest in the detergent industry in compositions for laundry applications which combine surfactants responsible for effective cleaning with additives which impart fabric softening and antistatic effects. Compositions of this type are described, for instance, in U.S. Pat. Nos. 4,058,489, 4,259,215, 4,264,457, 4,333,862, Japanese Pat. No. J55058257, and published patent application Ser. No. 2,054,635 of the United Kingdom.
It is known that the softening and antistatic properties of such detergent and fabric softener combination products are the result of the coating or deposition of the additive onto the fabric surface during the wash cycle. Also during the wash cycle, of course, tne detergent components of the composition must function to remove soils from the fabric and suspend them in the wash water. Not surprisingly. it is found that in coating the fabric the softening and antistatic additives may interfere with removal of soils from the fabric and/or cause the redeposition onto the fabric of soils which had previously been removed.
There is a need in formulating detergent compositions generally, and combinations of detergent and fabric softener in particular, to be able to distinguish between the soil removal capabilities of the detergent components and any interference with detergent action which may result from the use of the fabric softener. It is the object of this invention to provide a test method capable of the desired distinction.
One method has been proposed in the art for measuring "recontamination" of fabric during washing with fabric softening detergent products. The aforementioned U.S. Pat. No. 4,259,215 describes a test in which carbon black is ultrasonically dispersed in a solution of the detergent product in water. Clean cloth swatches are then washed in the dispersion. After washing, the swatches are evaluated to determine if they are darker then the original clean swatches. Although this patent points out the need for a soil redeposition test, the method which it describes for this purpose fails to fully satisfy that need in several respects. For example, the method of the patent does not measure recontamination or redeposition in literal terms, since the soil is initially present not on soiled fabric but in a controlled dispersion. For this same reason, the test is unable to simultaneously measure both soil removal and soil redeposition during the same wash cycle. Furthermore, the test as applied in the patent is qualitative rather than quantitative, and does not yield the type of result necessary for use in programs aimed at formulating products exhibiting optimum performance. Still further, this method of the prior art is limited to one type of soil. A full understanding of detergent performance requires the testing of a variety of soil types and/or mixtures of different soils.